A great many digital position control systems have heretofore been provided for moving an element along or about one or more axes in response to input numerical information. Typically, program information is supplied on punched or magnetic tape which is read to produce command pulses at rates corresponding to the desired speed of movement with each pulse corresponding to a certain increment of movement. One application is in machine tool control and an early disclosure relating thereto is contained in the McGarrell U.S. Pat. No. 3,079,522 issued Feb. 26, 1963. As described therein, each command pulse might correspond to a movement of one ten-thousandths of an inch and the command pulses may be applied at a rate such as to produce a maximum speed of one inch in five seconds or 0.2 inches per second, the frequency of the command pulses then being 2000 Hz.
In the McGarrell system, the command pulses are applied between and thereby added to pulses of a high frequency clock signal, or are used to delete pulses of the clock signal, the clock signal with the added or deleted pulses being applied through count-down circuits to produce a 200 Hz phase-modulated command signal which is applied to a phase detector along with a 200 Hz feedback signal from a resolver. The phase detector develops an analog error signal which is applied to a servo system to drive the resolver and a controlled element until a phase balance is obtained between the 200 Hz feedback and phase-modulated command signals.
Systems which are similar to that of the McGarrell patent are disclosed in British Pat. No. 1,171,690, the Kolell, et al. U.S. Pat. No. 3,490,017 issued Jan. 13, 1970, the McGee U.S. Pat. No. 3,539,895 issued Nov. 10, 1970 and the Sommeria U.S. Pat. No. 3,786,333 issued Jan. 15, 1974. In the system of the British, Kolell, et al. and McGee patents, the resolver is supplied with a 500 Hz signal and a feedback signal at that frequency therefrom is shaped into a square wave and compared with a square wave phase-modulated command signal at the same frequency to produce a signal which is passed through a low pass filter or integrator to produce an error signal. In the Sommeria system, the resolver is supplied with a 3000 Hz signal and a feedback signal at that frequency therefrom is shaped into a square wave and it and a square wave phase-modulated command signal are applied through count-down circuits both having a radix of eight to produce 375 Hz signals which are compared to produce an error signal for application to a servo drive.
Such systems and other similar systems using resolvers have been used very extensively in a wide variety of applications. Another prior type of system uses a shaft position encoder which is mechanically coupled to an element to generate pulses in response to movement of the element, each pulse corresponding to an incremental distance of movement. With encoders which use optical or other special techniques, it is possible to reduce the incremental distance which corresponds to one pulse to a very small value and thus obtain a high degree of accuracy. However, such encoders present many mechanical and other problems which preclude their use in many applications and especially where there is a requirement for a rugged system which will reliably operate at high speeds.
With regard to the performance capabilities of prior systems, the maximum frequency of command pulses which can be accepted by commercial systems has been on the order of 100 KHz, possibly up to 125 KHz in some systems, it being noted that the maximum usable command pulse frequency is a useful criterion for evaluation. Specifications as to operational speeds or obtainable accuracies are not always meaningful since by judicious selection of gear ratios or other parameters, it is generally possible to make improvements with respect to speed at the expense of accuracy or vice versa.
One recognized problem in attempting to increase the usable command pulse frequency for increased operational speed and/or accuracy is that the response characteristics and gains of servo motors and servo amplifiers have been limited. Improvements of such components have been made but there appear to be practical limits on making further improvements without going to undue expense or making sacrifices with respect to reliability, size, power requirements or other factors.